This invention relates to a torque sensor of noncontact type and, more particularly, to a torque sensor for converting the torque of a shaft into an electric signal.
Recently, systems for generating torque, e.g., an engine system, are required to detect a torque and produce a detection signal corresponding to the detected torque for controlling the torque generation system. Various torque sensors for detecting a torque of a shaft have been developed to meet the demand noted above. Particularly, there have been proposed torque sensors, which can measure the torque of a shaft accurately in a state out of contact with the shaft, as disclosed in William J. Fleming and Paul W. Wood, "Noncontact Miniature Torque Sensor for Automotive Application" SAE paper 8206206, presented at the Automotive Engineering Congress, K. Harada, I. Sasada, T. Kawajiro, M. Inoue, "A New Torque Transducer Using Stress Sensitive Amorphous Ribbon", IEEE Transactions on Magnetics, Vol. MAG-18, NO. 6, November 1982, and Japanese Patent Disclosure (KOKAI) No. 57-211030 (corresponding U.S. application Ser. No. 268,890 filed June, 1, 1984). The torque sensor provided by William J. et al. in "Noncontact Miniature Torque Sensor for Automotive Application" measures torque applied to an engine crankshaft by making use of the fact that a magnetic characteristic, i.e., magnetic permeability, of the crankshaft is varied according to the torque applied thereto. However, the magnetic property of the measurement zone of the engine crankshaft is not uniform, and there is magnetic anisotropy on the surface and the inside of the crankshaft. Therefore, the torque cannot be measured accurately. In addition, since the engine crankshaft itself does not have high magnetic permeability, it is necessary to generate a magnetic flux sufficient to penetrate the crankshaft measurement zone and be detected. Therefore, the magnetic flux control means is inevitably large in scale. In the torque sensor disclosed by K. Harada et al. M. Inoue in "A New Torque Transducer Using Stress Sensitive Amorphous Ribbons" and Japanese Patent Disclosure (KOKAI) No. 57-211030, an amorphous magnetic ribbon is arranged along the entire circumference of a portion of a shaft, and a coil assembly is provided around and coaxially with the shaft. The coil assembly generates a magnetic flux parallel to the axis of the shaft, and the permeability of the amorphous magnetic ribbon that is changed, according to the torque coupled to the shaft, is measured. Although this torque sensor can solve the problem noted above, since the coil assembly generates a magnetic flux parallel to the axis of the shaft, which has a relatively high magnetic reluctance, a comparatively large exciting current has to be supplied to the coil assembly. In addition, since the coil assembly is provided around the shaft, space for providing the coil assembly is necessary around the shaft. Therefore, the torque sensor cannot be readily assembled in the system which generates the torque, and depending on systems, the space for assembling the torque sensor cannot be ensured. Further, the amorphous magnetic ribbon arranged along the entire circumference of the shaft has to be given an induced magnetic anisotropy in a predetermined direction. However, it is difficult to give the amorphous magnetic ribbon arranged along the entire circumference of the shaft, i.e., a cylindrical amorphous magnetic ribbon, an induced magnetic anisotrophy in a predetermined direction. Further, the magnetic permeability of the amorphus magnetic ribbon arranged along the entire circumference of a shaft may not be uniform when the shaft is made of a Fe system. Variations of the magnetic permeability are liable to result without variations of the torque while the entire circumference of the shaft is under measurement due to lack of uniformity of the magnetic property of the shaft. Therefore, noise is introduced into the torque detection output, and the signal-to-noise ratio is reduced.